My 1969 D18's neck is still as straight as the day I bought it. I was led to believe by Bob Bigelow, who worked at Martins at the time, that there were a number of guitars that had the T-bar reinforcement after the nominal changeover date (up to 1970), including a 1969 D35 that Bob owned until he sold it to a friend of mine, who later sold it to Roger Whittaker.

As far as I know the only way to tell which reinforcement is present is to Xray the neck or destroy it, so I'll settle for knowing that it works.

My SPD16 and DM12 both have adjustable reinforcement, but I hope never to need to make any adjustment.